Alloy



Patented Sept. 8, 1936 1 UNITED STATES PATENT OFFICE rum '1'. Stroup, New Kensington, m, assignor to Aluminum Company of America, Pittsburgh, Pa... a corporation of Pennsylvania No Drawing. Application February 2, 1935, Serial No. 4,694

3 Claims. (CL 75147) This invention relates to a method of inhibiting oxidation in copper-free aluminum-base alloys when they are exposed to elevated temperatures designed to improve their physical properties.

5 During the fabrication of aluminum-base alloys to desired form and in the eventual thermal treat-' ment, the metal is subjected to temperatures considerably above room temperature for the purpose of altering its physical properties. The pe- 1 riod of exposure to the elevated temperatures varies with the size of the article treated and the number of articles charged to the furnace in a particular load. Where the article is of relatively thick cross section a longer time is required to 15 thoroughly heat it than where an article of thin section is treated. Again, where a large load is' charged into a furnace, the metal on the exterior of the pile of articles will become heated sooner,

- and be subjected to the elevated temperatures for 20 a longer period of time than the metal at the center of the charge. When so exposed to a high temperature for a long period of time in the presence of the ordinary atmosphere, certain alloys react with the heated atmosphere with the resultant formation of non-metallic substances. The attack is particularly insidious where it penetrates between the grains of metal thus reducing the effective cross section of sound metal and weakening the article. While the ex- 3 act mechanism of the reaction is not known, nor

has the composition of the product been definitely established, both process and product appea to be the result of oxidation and they will therefore be herein referred to as such for the sake of con- 35 venience in terminology.

One of the objecg of my invention is to inhibit the tendency of copper-free aluminum-base alloys to oxidize under the foregoing described conditions. A particular object is to reduce the oxi- 40 dation of aluminum-base alloys containing magnesium and silicon when the same are thermally treated. Another object is to effect this improvement without resorting to the use of special equipment or heat treating practices.

I have discovered that these objects may be achieved through the addition of a very small amount of lithium to the copper-free aluminumbase alloys. For example, I have found that the addition of about 0.1 per cent lithium to an alu- 50 minum-base alloy containingabout 0.6 per cent magnesium, and 1 per cent silicon reduced oxidation of the alloy when heated for 16 hours at 960 F. as'shown by an elongation which is27 2 per cent of that of the same alloy without lithium. The elongation is a reliable test of the progress of oxidation since the formation of oxidation product between the grains embrittles the alloy, with a consequent reduction in the elongation.

The members of the chemical group of alkali metals other than lithium, especially sodium and 5 potassium, have been employed in the production of aluminum-base alloys These metals have been especially useful in-altering the structure of aluminum-silicon alloys. The addition of these same alkali metals to the herein described alloys does not, however, diminish oxidation at elevated temperatures. Lithium as a member of the alkali metal group behaves in an altogether unique manner in this respect, it having a beneficial effect whereas the other elements of the group have either a neutral or adverse influence upon the tendency to oxidize at high temperatures.

For the purposes of my invention the presence of from about 0.005 to 0.15 per cent lithium in the alloy is generally sumcient to inhibit oxidation. Under particularly severe conditions as much as 0.25 per cent of this-element may be used. The

lithium may be added either in metallic form or as the carbonate; other methods of incorporating the element in the alloy have not proven to be satisfactory. The addition of this element is preferably made shortly before the alloy is poured rather than at an early stage in the melting procedure.

The addition of lithium to the alloy aflo'rds protection against oxidation where the alloy is heated between about 600 and 1050 F. for periods upv to about 20 hours. The beneficial effect of lithium diminishes beyond these limits of temperature and time. Aluminum-base alloys consisting of aluminum, about 0.5 to 4 per cent magnesium, and 0.5 to 4 per cent silicon are subject to oxidation on continued exposure to elevated temperatures, and I have found that the addition of lithium to this 0 type of alloy greatly reduces the tendency to oxidize. The presence of magnesium appears to induce the oxidation since alloys without this element seem to suffer less when heated for any length of time. As an example of the benefit to be derived from the use of lithium, the case of heating an aluminum-magnesium-silicon alloy maybe cited. The normal alloy contained about 0.6 per cent magnesium and 1.0 per cent silicon. When heated at 960 F. for Y4 hour and quenched, 60 the alloy had a tensile strength of 35,000 pounds per quare inch, a yield strength of 17,600 pounds per square inch, and an elongation of 24.5 per cent in 2 inches. When heated at the same temperature for 16 hours and quenched,

. the elongation only 38.8 per cent.

the alloy lost about 34 per cent in tensile strength, 12 per cent in yield strength, and 77.5 per cent in elongation. The same alloy containing 0.1 per cent of added lithium under the same heat treatment had an initial tensile strength of 36,900 pounds per square inch, a yield strength of 20,000 pounds per square inch, and an elongation of 24.5 per cent. After heating at 960 F. for 16 hours, the tensile strength dropped about 14 per cent, the yield strength about 11 per cent and The foregoing data clearly reveal the beneficial efiect of lithium in hindering oxidation.

Although oxidation is not entirely prevented, it is greatly reduced with a consequent improvement in the properties of the alloy as compared with the normal product under similar conditions, a result not heretofore obtained in commercial operations. Furthermore, the beneficial eflect of lithium becomes more pronounced as the length of the exposure increases. Lithium does not provide protection to the alloy for an indefinite time at elevated temperatures, but it does cause a marked reduction in attack for periods of time employed in thermally treating aluminum-base alloys.

The term aluminum as herein employed refers to the commercially available metal which contains the usual impurities of iron and silicon, although the silicon later becomes an intentional alloying constituent in the final product.

I claim:

1. An aluminum-base alloy consisting oi aluminum, about 0.5 to 4 per cent magnesium, 0.5 to 4 per cent silicon, and 0.005 to 0.25 per cent lithium, said alloy being characterized by resistance to oxidation between about 600 and 1050 F. when exposed to said temperatures for periods up to about 20 hours.

2. An aluminum-base ally-consisting of aluminum, from about 0.5 to 4 per cent magnesium, 0.5 to 4 per cent silicon, and 0.005 to 0.15 per cent lithium, said alloy being characterized by resistance to oxidation when the alloy is heated to between 600 and 1050 F. for a period up to about 20 hours.

3. An aluminum-base alloy composed of about 0.6 per cent magnesium, 1 per cent silicon, 0.1 per cent lithium and the balance aluminum, said alloy being characterized by resistance to oxidation when heated to between 600 and 1050 F. for a period up to about 20 hours.

PHIIJP T. B'I'ROUP. 

